Organic silanes having alkoxy or aryloxy substituents, and amino-substituted alkoxysilanes in particular, find use in a variety of applications such as in laundry additives, in caulking compound formulations, and as coupling agents between inorganic and organic surfaces such as in fiberglass products. Normally these silane compositions have a very pale yellow coloration.
Organic silanes having alkoxy or aryloxy substituents are typically prepared by reacting a halosilane with an alcohol or a phenol, commonly followed by treatment with an acid acceptor to neutralize residual acidic halide by-product. Amino-substituted alkoxysilanes can be prepared by the known reaction of a haloalkylalkoxysilane with a primary amine or a secondary amine. An acidic halide, such as hydrogen chloride, is also formed as an undesirable by-product of this reaction, and is typically removed as the hydrogen halide salt of an acid acceptor. As used herein the term "acidic halide" encompasses free residual hydrogen halide, bound hydrogen halide such as amine hydrohalide, silanic halide and mixtures thereof. Acidic halide contamination of the silane product is undesired for a variety of reasons, including increased corrosion of surfaces to which it is applied.
Many of the commonly-used inorganic alkaline neutralizing agents, such as the alkali metal hydroxides, are not used to remove acidic halide contaminants from silanes, particularly in the case of acidic halide contamination of alkoxysilanes. Such inorganic agents are not used because water is produced by the neutralization reaction and contributes to silane product degradation via a hydrolysis mechanism. The same is true for sodium carbonate and sodium bicarbonate which have been used in the past for neutralizing certain organosilanes. By-product water is a particularly troubling problem for alkoxysilanes having relatively high molecular weights since these alkoxysilanes produce a higher level of impurities when they undergo hydrolysis.
In the past, acidic halide contamination of alkoxysilanes generally, and particularly of amino-substituted alkoxysilanes, has been controlled by a post-reaction treatment with a strong base such as a metal alkoxide, e.g., sodium methoxide. Unfortunately, the quality of the silane product can be adversely affected by the level of metal alkoxide addition. If an insufficient amount of the metal alkoxide is added, an undesirably high residual halide level is encountered in the silane product. On the other hand, the addition of even a small excess of the metal alkoxide commonly causes an unacceptably severe and irreversible color development in the alkoxysilane product, particularly in those products having amine substitution. Such coloration is thought to be due to oxidation of the amine in the presence of the excess base. In addition, certain alkoxysilanes, such as vinyl alkoxysilanes, in the presence of metal alkoxide, in addition to color development, can also violently decompose during subsequent thermal treatment (e.g., distillation).
In light of the above, great care is exercised to obtain a proper neutralization end point when metal alkoxides are used for acidic halide removal. This degree of care is very inconvenient in an industrial context. Implementation of metal alkoxide neutralization, therefore, tends to be very time-consuming and often leads to the uneconomical reworking, e.g., distillation, or in the extreme, discarding of over-neutralized products.
One method which allows for the use of metal alkoxide without the need for assiduous control of the neutralization end point is described in U.S. Pat. No. 5,084,588. By the patented method for reducing the level of acidic halide contamination in alkoxysilanes, a metal alkoxide is provided to the system during a first stage treatment (partial neutralization) at a concentration of less than one molar equivalent based on the acidic halide content of the alkoxysilane. An alkali metal or alkaline earth metal salt of a weak acid is then used in excess to neutralize the remaining acidic halide and buffer the system without overneutralization. Suitable weak acids are those having a dissociation constant (Ka) between 10.sup.-15 and 10.sup.-2.
Notwithstanding the advance in the art made by said U.S. Pat. 5,084,588, there is a continuing need for a process which allows for the use of an excess of a basic reagent such as metal alkoxide and which readily achieves the desired neutralization of acidic halide without adversely affecting alkoxysilane product quality.